swift234

alloc_stack needs to return two values like alloc_box.

The current implementation of dealloc_stack in IR-gen is a
no-op, but that's very much wrong for types with non-trivial
local allocation requirements, e.g. archetypes.  So we need
to be able to do non-trivial code here.  However, that means
modeling both the buffer pointer and the allocated address
in SIL.

To make this more type-safe, introduce a SIL-specific
'[local_storage] T' type that represents the required
allocation for locally storing a T.  alloc_stack now returns
one of those in additon to a *T, and dealloc_stack expects
the former.

IR-gen still implements dealloc_stack as a no-op, but
that's now easy to fix.

Swift SVN r6937